The invention relates to a low-pressure mercury discharge lamp provided with a discharge vessel which encloses a discharge space in a gastight manner and in which a filling comprising mercury is accommodated, and with means for maintaining an electric discharge in the discharge space. The discharge vessel is provided with a luminescent layer, generating light during operation with a spectrum which is substantially composed of radiation in a first wavelength region from 590 to 630 nm, in a second wavelength region from 520 to 565 nm, and in a third wavelength region from 430 to 490 nm.
Low-pressure mercury discharge lamps are widely used for general lighting purposes. The radiation generated in the discharge space is mainly ultraviolet radiation which is converted into visible radiation by the luminescent layer. The discharge may be maintained by electrodes between which a potential difference obtains and which may be arranged inside or outside the discharge space. Alternatively, the discharge may be maintained by means of a coil which generates a high-frequency magnetic field in the discharge space during operation, or the means for maintaining the discharge may be formed by a microwave generator.
Standard lamps are mostly used in surroundings where color rendering plays only a limited part, such as industrial lighting and street lighting, in which case the luminescent layer is composed of halophosphates such as calcium halophosphate activated by Sb and Mn. This is because of the low cost price of halophosphates. Such lamps have a mediocre color rendering (Ra=50 to 60) and a reasonable luminous efficacy (.eta.=80 lm/W). Ra represents the general color rendering index as defined in CIE Publication No. 13, 1965, of the Commission Internationale de l'Eclairage.
Low-pressure mercury discharge lamps having a high luminous efficacy are disclosed in U.S. Pat. No. 4,075,532. The luminescent layer here has an emission spectrum with a narrow blue and a wide yellow emission band. A disadvantage of these lamps is that the color rendering is substantially worse than that of the standard lamps mentioned earlier.
W.A. Thornton, Three Color Visual Response, J. Opt. Soc. Am. Vol. 62, No. 3, 457-459, states that the spectral colors adjacent 450, 540, and 610 nm contribute most to the color perception and that on the contrary wavelengths adjacent 500 and 580 nm are unfavorable.
A lamp of the kind mentioned in the opening paragraph is known from U.S. Pat. No. 3,937,998. Luminescent materials activated by rare earth metals are used in the luminescent layer of this lamp on account of the generally comparatively narrow emission spectra of these materials. Emission in the wavelength region from 520 to 565 nm is generated here by means of aluminates activated by terbium. Furthermore, bariummagnesium aluminate activated by bivalent europium and various other luminescent materials suitable for emission in the wavelength region from 430 to 490 nm are mentioned therein. Yttrium oxide activated by trivalent europium, whose emission spectrum has a maximum at 611 nm and a half-value width of 2 nm, serves as a luminescent material here for emission in a wavelength region from 590 to 630 nm. Such light sources have a good color rendering (Ra.apprxeq.80) and a high luminous efficacy (.eta..apprxeq.90 lm/W).
It is noted that a luminescent material for the 430 to 490 nm wavelength region is not always necessary in lamps of this kind. The visible radiation emitted by the mercury discharge in that wavelength region forms a sufficient contribution especially in lamps whose emitted radiation has a low color temperature.
As described in U.S. Pat. No. 4,335,330 (Peters et al.), many of such luminescent materials activated by rare earth metals are suitable for lamps with high wall loads (&gt;500 W/m.sup.2), provided certain conditions are complied with.
A disadvantage, however, is the comparatively high cost price of luminescent materials activated by rare earth metals as compared with the standard lamps mentioned further above, which renders the known lamp unattractive in those applications where a mediocre color rendering can suffice.